A continuation of a study of the photochemistry of urocanic acid (UA) is outlined, and a new program involving the photochemistry of a highly phototoxic family of drugs, the tetracyclines, is presented. The UA project will focus on UA interactions with thymine, DNA and singlet oxygen, to determine UA's potential as a photomutagen as well as a photoprotective agent. A complete study of tetracycline photochemistry, both unimolecular and in conjunction with biological molecules, is proposed; all products will be tested as potential carcinogens.